This invention relates to the detection of small particles of radioactive materials such as plutonium oxide in the air. More particularly, this invention relates to an apparatus for concentrating particles of radioactive materials such as chemical combinations of selected isotopes of uranium, plutonium, americium and the like from a stream of air containing various of the radioactive isotopes described and also containig radioactive dust that emits alpha particles in a range that will comprise background noise in an instrument reading the alpha activity of the particles described above.
Certain isotopes of uranium and some of the transuranic elements emit radiation that must be monitored in the event of any escape of gases containing these elements. The maximum permissible concentrations of some of the transuranic isotopes over a 40-hour week are listed in Handbook 69 of the National Bureau of Standards. Certain information from that handbook is presented in Table I which lists for various isotopes the energy of alpha particles emitted, the half-lives and the maximum permissible concentrations.
TABLE I ______________________________________ IMPORTANT TRANSURANIC ISOTOPES WHICH EMIT ALPHA PARTICLES Alpha Particle MPC* Energy Half-Life a Isotope (Mev) (Years) (.mu.C/CC) ______________________________________ 236 Pu 5.77 2.85 2 .times. 10.sup.-12 5.72 238 Pu 5.50 86.4 2 .times. 10.sup.-12 5.46 239 Pu 5.16 24,390 2 .times. 10.sup.-12 5.11 240 Pu 5.17 6580 2 .times. 10.sup.-12 5.12 242 Pu 4.90 3.79 .times. 10.sup.5 2 .times. 10.sup.-12 4.86 241 Am 5.49 458 6 .times. 10.sup.-12 5.44 243 Am 5.28 7.95 .times. 10.sup.3 6 .times. 10.sup.-12 5.23 242 Cm 6.12 0.445 1 .times. 10.sup.-10 6.07 243 Cm 6.06 32 6 .times. 10.sup.-12 5.99 5.79 5.74 244 Cm 5.81 17.6 9 .times. 10.sup.-12 5.77 ______________________________________ *For a 40-hour week for the soluble form from NBS Handbook 69.
Because these concentrations are so small, it is important to be sure that there is a minimum amount of interference with measurements by radioactive dust which is defined as airborne particulate matter other than uranium and the transuranics that emits alpha particles in energy ranges of the order of those of uranium or the transuranics. If such radioactive dust is trapped on a filter in the course of accumulating plutonium or uranium for a measurement, the radiation from the radioactive dust distorts the measurement. It is possible to effect a certain level of discrimination by the use of multichannel analyzers which permit one to obtain a relatively broad spectrum of radiation and to gather therefrom information that permits some measure of correction for background. This is undesirable, however, as a continuing monitor in a nuclear power plant, a fuel reprocessing facility, a fuel fabricating facility, a laboratory for radiation research, or any other installation in which it is desirable to maintain a continuing watch for low levels of uranium and the transuranics in the air and to maintain a record of the amounts observed. It is difficult to justify the cost of a multichannel analyzer for effecting such discrimination if there is a better way to minimize the interfering counts from radioactive dust. Furthermore, even the use of a multichannel analyzer may not be adequate because of the spreading of the energy spectrum due to differing energy losses in the air space between a filter and a detector, and because the allowable maximum permissible concentrations are so low compared to typical background levels.
It is an object of the present invention to provide a better way of monitoring low concentrations of particulate uranium or transuranic isotopes in the air.
It is a further object of the present invention to provide a better apparatus for minimizing physically the presence of interfering background radiation in a stream of air containing small particles of uranium or transuranic elements.
Other objects will become apparent in the course of a detailed description of the invention.